In recent years, x-ray radiography had been actively used in a wide variety of fields, such as the applications in medicine field, industrial field, home security field and agricultural field, etc. In the field of medicine, there are three main demands, namely a two-dimensional (2D) imaging, a temporal-dynamic fluoroscope imaging and a three-dimensional (3D) imaging, and the medical instruments being used in those applications include medical x-ray imaging devices, medical fluoroscopy and three-dimensional computed tomography. It is noted that the two-dimensional (2D) imaging and the temporal-dynamic fluoroscope imaging are used primarily for generaying two-dimensional planar images for medical evaluation. Taking a 2D x-ray process for chest anterior/posterior view imaging for example, the so-acquired planar images lack depth information so that the organs in the chest may appear overlapped on each other in the x-ray images, resulting in illegible fine structures. Therefore, it is difficult to interpret the planar x-ray images for identifying whether a lesion is located in front or behind the heart, mediastinum, diaphragm, vertebralis, etc. On the other hand, the 3D imaging, such as the CT scan, is performed using the x-ray sources and detectors that are arranged concyclically and surrounding a patient so as to generate a number of successive slices of cross-sectional image of the patient that can be then collected and digitally “stacked” together to form a three-dimensional image of the patient for medical interpretationa and abnormality identification. Despite that CT scan is able to provide generate a number of successive slices of cross-sectional image of the patient for forming a three-dimensional image of the patient, its high cost and high-dose risk cause the CT scan to be used only as a second-line inspection tool in medical diagnosis. According to the Report no. 160 on population exposure released by the National Council on Radiation Protection and Measurements (NCRP) in 2006, Americans were exposed to more than seven times as much ionizing radiation from medical procedures as was the case in the early 1980s, i.e. from 3.1 mSv at 1980s to about 5.5 mSv at 2006. The report further indicates that The increase was primarily a result of the growth in the use of medical imaging procedures, as such exposure of medical imaging procedures had grown 6 times from 0.5 mSv to 3.0 mSv with the 25-year period which is positively proportional to the growing prevalence of x-ray devices and CT scan in American. Therefore, it is reasonable to assume the increase was due mostly to the higher utilization of computed tomography (CT) and nuclear medicine. Consequently, the recent focal point in radiographic technology is committed to minimize the risk of radiation dose without sacrificing the quality of the three-dimensional images and the resulting medical benefits.
In additional to the aforesaid 2D imaging and CT imaging, a new technique, i.e. digital tomosynthesis, had been developed recently, which is a method for performing high-resolution limited-angle tomography at radiographic dose levels. Digital tomosynthesis combines digital image capture and processing with simple tube/detector motion as used in conventional computed tomography (CT). However, though there are some similarities to CT, it is a separate technique. In CT, the source/detector makes at least a complete 180-degree rotation about the subject obtaining a complete set of data from which images may be reconstructed. Digital tomosynthesis, on the other hand, only uses a limited rotation angle with a lower number of discrete exposures than CT. This incomplete set of projections is digitally processed to yield images similar to conventional tomography with a limited depth of field. Because the image processing is digital, a series of slices at different depths and with different thicknesses can be reconstructed from the same acquisition. However, since fewer projections are needed than CT to perform the reconstruction, radiation exposure and cost are both reduced. Moreover, the method of digital tomosynthesis can be performed using the current medical x-ray imaging devices with some improvement, so that it is possible to fulfill the abovementioned three main imaging demands in the field of medicine in one x-ray imaging device.
However, due to the limited angle scanning design that is used in the conventional digital tomosynthesis, the 3D imaging to an internal object with directional structure may not be satisfactory. Experimentally when a digital tomosynthesis device with limited angle scanning design adopts a longitudinal-direction scanning arrangement that is similar to the conventional medical x-ray device, the loading mechanism for the x-ray tube in the digital tomosynthesis device is driven to move along the longitudinal direction of its image table for scanning and imaging so that the the moving direction of the loading mechansim is parallel to the growing direction of human carotid arteries, and thereby the so-obtained 3D images of the carotid arteries can be clear for identification. On the other hand, if the aforesaid digital tomosynthesis devie is used for imaging a possible skull fracture while the cracking direction of the skull fracture is orientated different from the moving direction of the forgoing loading mechanism, the so-obtained 3D imaging of the skull fracture may not be clear enough. Therefore, for scanning a skull fracture, it is preferred to had the loading mechanism to move translationally, i.e. the loading mechansim is enabled to move translationally and thus is moving perpendicularly to the forgoing longitudinal direction, so that the translational moving direction is able parallel to the cracking direction of the skull fracture for allowing a clear imaging of the skull fracture. Generally, the clearness of a 3D imaging obtained from a digital tomosynthesis device with limited angle scanning design can be greatly affected by the directional structure in the object to be scanned, and can be improved if the scanning direction is about parallel to the orientation of the directional structure in the object. Nevertheless, as the direction of growth for most tissues and organs in human body, such as human skeleton, airway structure and blood vessel, can be very complex, the conventional unidirectional scanning design is not sufficient for satisfying the needs for scanning different human portions.